Polymer hydrogels are widely used for contact lenses and a large number of soft contact lenses of this type are based on the polymers described in U.S. Pat. No. 3,220,960 (Wichterle). Wichterle's polymers are hydroxy (lower alkyl) methacrylates or acrylates, cross-linked with a small percentage of the corresponding diester e.g. ethylene glycol dimethacrylate (EGDM). Polymers based upon hydroxy ethyl methacrylate (HEMA) and cross-linked with EGDM can be hydrated to form clear hydrogels having good mechanical properties. Unfortunately the oxygen permeability of such lenses (which is directly related to the retained water content) is generally insufficient to allow the lenses to be worn continuously. Consequently, although soft lenses are usually more comfortable than hard lenses, most commercial soft lenses have to be removed at night in order to avoid the development of anoxic edema arising from insufficient corneal respiration.
One important exception is the soft lens marketed under the Registered Trade Mark "Permalens" by Coopervision (U.K.) Limited and described in British Pat. Nos. 1,475,605 and 1,385,677. While such soft contact lenses have been commercially successful there has been a continuing need to further improve the mechanical properties of the hydrogels, especially in relation to resistance to tear initiation and "toughness". In this specification the "toughness" of a hydrogel means the area between the stress-strain curve of the hydrogel and the strain axis. Tear initiation strength and toughness have been found from experience to be predictors of durability of a hydrogel considered for use as a contact lens material.
It has now surprisingly been found that substantial improvements in the mechanical properties of lenses of the kind generally described in the above patents can be secured by selection of a narrow range of monomer compositions and by effecting the polymerisation under particular conditions.
According to one aspect of the present invention there is provided a process for the preparation of a contact lens comprising a hydrogel having a water content from about 50% to 85% by weight of water, which process comprises polymerizing, in the presence of an azo type initiator, a monomer mixture comprising from 25 to 50% by weight of N-vinyl pyrrolidone, up to 5% by weight of methacrylic or acrylic acid, and 0.1 to 1% by weight of 1,3,5-tris(propenoxy)-2,4,6-triazine (TPT) or perhydro-2,4,6,-tris (propene-2)-1,3,5,-triazine (T.T.P.T.), as a cross-linking monomer, the balance being essentially hydroxy alkyl acrylate or methacrylate, wherein the alkyl group is ethyl or propyl.